IJRET: International Journal of Research in Engineering and Technology
eISSN: 2319-1163 | pISSN: 2321-7308
LOAD FREQUENCY CONTROL IN CO-ORDINATION WITH
FREQUENCY CONTROLLABLE HVDC LINKS USING FUZZY LOGIC
CONTROLLER
Ajitha priyadarsini .S1, P. Melba Mary2
1
Asst.professor, EEE, Narayanaguru College of Engineering, Tamil Nadu, India, ajitha_priya03@yahoo.com
2
Principal, V. V. College of Engineering, Tamil Nadu, India, principal@vvcoe.org
Abstract
In this paper decentralized load frequency control (LFC) for suppression of oscillations in multi-area power systems using fuzzy logic
controller was studied. A three area system is considered in which areas 1 and 2 and areas 1 and 3 are connected by HVDC
transmission links and areas 2 and 3 are connected by normal AC tie-line. The performance of the fuzzy logic controller is compared
with the conventional PI controller and the simulation results shows that fuzzy logic controller is very effective enhancing better
damping performance in non-linear conditions.
Keywords: Load Frequency Control, High Voltage Direct Current transmission Link, Proportional Integral Controller,
Fuzzy Logic Control.
-----------------------------------------------------------------------***-----------------------------------------------------------------------1. INTRODUCTION
For the all-round development of any country and industrial
development, Electrical energy is an essential ingredient. This
is obtained by using a large-scale power interconnection
system, which is intended to make the electrical energy
generation and transmission more economical and reliable.
Hence, in modern large interconnected system controllers are
set for a particular operating condition and they take care of
small changes in load demand without frequency and voltage
exceeding the prescribed limits.
A power system consisting of number of areas are
interconnected by means of AC tie-lines or HVDC
transmission links.[1] It is necessary to maintain the frequency
constant so that the power stations run satisfactorily in parallel
and various motors operating in the system run at the desired
speed. By achieving a balance between the generation and the
connection of load, the frequency can be made constant to a
major extent [2]. The frequency of a power system entirely
depends upon the speed at which the generators are rotated by
their prime movers. A constant speed is obtained by adjusting
the gate or control valve opening of the speed governor. Power
system operation at low frequency than that specified
maximum permissible change in frequency ( ± 0.5Hz) causes
certain serious problems. When operating at frequencies below
49.5Hz, some types of steam turbines undergo excessive
vibrations. In certain turbine rotor the effect causes resultant
metal fatigue and blade failures. When the frequency falls
below 49Hz, the turbine regulating devices fully open and the
generating units becomes completely loaded. As the frequency
decreases, the generator exciter loses their speed and generator
emf falls and the voltage in the power system unit drop. This
brings the danger of a “voltage avalanche” and disconnection
of the consumers [3]. The main requirement of load frequency
control is to ensure that the frequency of the various bus
voltage and current are maintained at or near specified nominal
value. Also to ensure that the tie-line power flows among the
interconnection areas are maintained at specified levels[4,5].
Load frequency control made the operation of interconnected
systems possible and today it is still the basic of any advanced
concept for the guidance of large systems.
Power systems are generally non-linear. Controllers designed
based on linear techniques like PI,PID etc. work satisfactorily
on linear conditions only and a load frequency controllers
designed on these techniques are not adequate for a power
system[6]. Conventional PI control approach used in this
project achieves zero steady state error in frequency but
exhibits relatively poor dynamic performance by large
overshoots and settling time. In recent year’s AI based
approaches have attracted considerable attention for control
strategies. First fuzzy logic based approach is used. A fuzzy
logic controller uses fuzzy logic as a design methodology
which can be applied, in developing linear and non-linear
systems. In this paper, advantages of fuzzy logic controller [7-
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IJRET: International Journal of Research in Engineering and Technology
9] have been employed to design a LFC to maintain desired
megawatt output of generator unit and assist in controlling the
frequency of three area power system and provide good
damping over wide range of operating conditions.
eISSN: 2319-1163 | pISSN: 2321-7308
∆F ( s ) = [∆PG ( s ) − ∆PD ( s )] ×
KP
1 + sTP
(2)
2.4 Modeling the Tie-Line
2. SYSTEM MODELING
Modeling of physical system is an important task of the power
system design procedure. The first step in the analysis of a
dynamic system is to derive its mathematical model.
Mathematical model may assume different forms. Once a
mathematical model of a system is obtained various analytical
and computer tools can be used for analysis and synthesis
purposes.
Consider the system which has three areas as shown in fig..1..
These areas are interconnected by frequency-controllable
HVDC links or normal tie-lines. Areas 1 and 2 and Areas 1
and 3 are interconnected by HVDC tie-lines and Areas 2 and 3
are interconnected by normal tie-line.
2.1 Speed Governing System Model
Area 1
If a generating unit is operated with fixed mechanical power
output from the turbine, the result of any load change would be
a speed change sufficient to cause the frequency sensitive load
to exactly compensate for the load change. This condition
would allow system frequency to drift for outside acceptable
limits. This is overcome by adding a governing mechanism
that senses the machine speed and adjusts the input valve to
change the mechanical power output to compensate for load
changes and to restore frequency to nominal value.


Kg
1

∆F ( s ) *
D
 1 + sTg
HVDC tie-line
Area 2
Area 3
Normal tie-line
Fig1. Three Area Systems
The standard transfer function for a normal tie-line, which
shows the incremental power flow from the ith area to the jth
area, is given by
Equation of the speed governor model,
∆X(S)=  ∆ΡC ( S ) −
HVDC tie-line
(1)
∆Ptieij =
K Aij
s
(∆f i − ∆f j )
(3)
2.2Turbine Model
Prime-movers are devices which convert any form of energy
into a mechanical energy. There is incremental increase in
turbine power, due to the change in valve position that will
result in an increased generator power.
G(s) =
ΔPG (s)
Kt
=
ΔX(s) 1 + sTt
In considering the frequency controllable HVDC links, it is
assumed that the deviation in the power system voltage, under
the context of load-frequency control is negligible. Simple
first-order AFC models, in the form of KHij /(sTHij+1), can
therefore be used. The transfer functions of the HVDC links,
in the presence of the AFCs, are given by
∆Ptieij =
2.3 Generator Load Model
The synchronous machine as an ac generator driven by a
turbine is the device, which converts mechanical energy into
electrical energy. An isolated generator is only supplying local
load and is not supplying power to another area via tie line.
Suppose there is a real load change ∆PD , due to the action of
turbine controllers, the generator increases its output by the
amount ∆PG .
∆Ptieij =
K Hij
sTHij + 1
− K Hij
sTHij + 1
∆f i
(4)
∆f j
(5)
A tie-line frequency bias control scheme is normally adopted
in each area where the area control error (ACE) has to reach
zero asymptotically.
ACE for the ith area is given by
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IJRET: International Journal of Research in Engineering and Technology
ACEi (t) = KBi ∆f i (t ) + ∆Ptiei (t )
(6)
2.5 Integral Control
The signal generated by the integral control must be of
opposite sign to ‘f(s)’. The steady state change in frequency
has been reduced to zero by the addition of the integral
controller. Change in frequency reaches steady state only when
change in generated power equal to change in demand equal to
constant. Because of the integrating action of controllers, this
is only possible if change in frequency is equal to zero. In
central load frequency control of a given control area, the
change (error) in frequency is known as Area Control Error
(ACE). By using the control strategy shown in fig 2., we can
control the intolerable dynamic frequency changes with change
in load.
Kg
1+sTg
Controller
B
Kt
1 + sTt
eISSN: 2319-1163 | pISSN: 2321-7308
The decision making logic is the kernel of an FLC. It has the
capability of simulating human decision making based on
fuzzy concepts and of inferring fuzzy control actions based on
the defined rules
3.1 Fuzzification and Membership Functions
Fuzzification implies the process of transforming the crisp
control values of the inputs to a controller, to fuzzy domain.
Selection of the control variables relies on the nature of the
system and its desired output. Seven linguistic variables for
each input variables were used to get the desired performance.
The linguistic variables are specified by Gaussian membership
function.
Membership function forms a crucial part in a fuzzy rule base
model because they only actually define the fuzziness a control
variable or a process variable. A set of membership defined for
seven linguistic variables are NB, NM, NS, ZE, PS, PM, PB
respectively is shown in fig.3 and fig .4
KP
1 + sTP
1
D
Fig2. Block diagram of Single Area Load Frequency Control
3. FUZZY LOGIC CONTROLLERS: A BRIEF
Fig3. Membership function for frequency deviation
OVERVIEW
Fuzzy logic is a novel approach that incorporates an alternate
way of thinking which allows one to model complex systems
using a higher level of abstraction. It is a very powerful
method of reasoning when mathematical models are not
available and input data are imprecise. It also finds extreme
application wherever a logic in the spirit of human thinking
can be introduced. In fuzzy logic, a particular object has a
degree of membership in a given set which is in the range of 0
to 1. A Fuzzy Logic Controller (FLC) uses fuzzy logic as a
design methodology which can be applied in developing linear
and non-linear systems. FLC techniques have been found to be
a good replacement for conventional control techniques, which
require highly complicated mathematical models. The FLC
comprises of four principal components.
1) A fuzzification interface
2) A knowledge base
3) A decision making logic and
4) A defuzzification interface
Fig 4.Membership function for integral of frequency deviation
3.2 Rules Creation and Inference
In general fuzzy systems map input fuzzy sets to output fuzzy
sets. Fuzzy control rules provide a convenient way for
expressing control policy and domain knowledge. The proper
choice of process state variables and control variables are
essential to the characterization of the operation of a fuzzy
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IJRET: International Journal of Research in Engineering and Technology
eISSN: 2319-1163 | pISSN: 2321-7308
system. The modes of deriving fuzzy rules are based on trial
and error method. Since each of the input variables contain
seven linguistic variables, as a result of 49 rules are devised.
Table -1 Fuzzy Rule Base
Fig5. Membership function for control signal
4 .SIMULATION RESULTS
In the three area system described in this paper, Areas 1 and 2
run on reheat type turbines; Area 3 uses non-reheat type
turbine. Areas 2 and 3 are interconnected by a normal tie-line.
It is assumed that both areas 2 and 3 may face large load
changes that will cause a significant and unacceptable
frequency deviation. They are therefore linked to Area 1 via
HVDC tie-lines for assistance in frequency regulation.
Conventional PI controller anf fuzzy logic controller is used to
control the intolerable dynamic frequency changes with change
in load. Matlab software is used for simulation study.
The details of the test system are taken from [1] and consist of
three areas. The first two areas uses reheat type turbines and
the last one uses non-reheat type turbines. Area 1 assists in the
frequency regulation for Areas 2 and 3 via HVDC links with
AFCs. Areas 2 and 3 are connected with a normal tie-line.
The rule base contains the fuzzy IF-THEN rules of Mamdani
type. An example of an ith rules is,
If ∆f is NB AND ∫∆f is ZE THEN ∆Pc is NB.
In the inference method, we wish to deduce or infer a
conclusion, given a body of facts and knowledge. The modes
of deriving fuzzy inference are based on the min- max method.
The minimum of the two antecedent conjunct’s value is chosen
to be the consequent (wi). For example if ∆f belongs to NS
with a membership of 0.5 and ∫∆f belongs to ZE with a
membership of 0.9 then the rule consequent (wi) will be 0.5.
Various disturbances under different operating conditions are
applied to the test system.
Case Study 1
When a 0.05p.u step increase in torque reference is applied at
0sec to Area 1 and Area 2 of the Three Area Load Frequency
Control System and also a -0.05p.u step increase in torque is
applied at 0sec to Area 3 of the Three area Load Frequency
Control System, the frequency response obtained is shown in
figure 6.. In case study1 all the three areas are under lowest
parameter condition.
3.3 Defuzzification
Defuzzification is the process of converting fuzzy quantity to a
precise quantity. The output of the fuzzy process can be the
logical union of two or more fuzzy membership functions
defined on the “universe of discourse” of the output variable.
The system has two inputs and a single output which implies
two input gains K1, K2 and output gains K3. In the present
case, the output gain was set to give the maximum allowable
control action while the other two parameters K1&K2 were
used for tuning.
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IJRET: International Journal of Research in Engineering and Technology
eISSN: 2319-1163 | pISSN: 2321-7308
Case Study 3
When a 0.05p.u step increase in torque reference is applied at
0sec to Area 1 and Area 2 of the Three Area Load Frequency
Control System and also a -0.05p.u step increase in torque is
applied at 0sec to Area 3 of the Three area Load Frequency
Control System, the frequency response obtained is shown in
figure 8.. Here area 1 is under nominal parameter condition,
area 2 is under lowest parameter condition and area 3 is under
highest parameter condition
Fig6. Area responses for case study 1
Case Study 2
When a 0.05p.u step increase in torque reference is applied at
0sec to Area 1 and Area 2 of the Three Area Load Frequency
Control System and also a -0.05p.u step increase in torque is
applied at 0sec to Area 3 of the Three area Load Frequency
Control System, the frequency response obtained is shown in
figure 7. In case study 2 area 1 is under lowest parameter
condition, area 2 is under highest parameter condition and area
3 is under nominal parameter condition.
Fig8. Area responses for case study3
CONCLUSIONS
In this paper fuzzy logic is proposed to tune the parameters of
PI controller. A three area power system is considered to
demonstrate the proposed method… Use of HVDC links in the
system reduces long distance transmission cost, also power
loss is less. The simulation studies with fuzzy logic based LFC
and PI controller based LFC under various disturbances shows
that the fuzzy logic based LFC is very effective enhancing
better damping performance in non-linear conditions.
REFERENCES
Fig7. Area responses for case study 2
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[3]D. P. Kothari, I. J. Nagrath,Modern power system analysis
Tata McGraw-Hill Publishing Company Limited (2003)
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IJRET: International Journal of Research in Engineering and Technology
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eISSN: 2319-1163 | pISSN: 2321-7308
BIOGRAPHIES:
Mrs. Ajitha priyadarsini.S. (born March 11, 1979) is a
native of Marthandam, Tamil Nadu. She took her B.Tech in
Electrical
and
Electronics Engineering from the
Govt.Engineering College Thrissur, M.E in Power Systems
Engineering from the Arulmigu Kalasalingam College of
Engineering, Srivilliputhur in 2000 and 2005 respectively.
Currently she is pursuing P.hd in Anna University. She has
eight years of teaching experience and two years of industrial
experience. She has presented papers in conferences.
Currently, she is working as an Asst.professor in
Narayanaguru College of Engineering, Manjalumoodu. Her
current research interests include power systems..
Prof. Dr. Melba Mary. P (born October 7, 1969) is a native
of Tirunelveli, Tamil Nadu. She took her B.E in Electrical and
Electronics from The Government College of Engineering,
Tirunelveli, M.E in Control and Instrumentation from the
College of Engineering, Guindy, Anna University, Chennai in
1990 and 1995 respectively. She worked as lecturer in
National Engineering College, Kovilpatti and as Professor and
Head of Electrical and Engineering department at National
College of Engineering, Maruthakulam. She has few years of
industrial experience too. Dr. Melba Mary’s main research
centers on hybrid control system theory, industrial process
control, robotics and various intelligent techniques used in the
design of controllers. She has presented papers in various
conferences and published many papers in National and
International journals. She is a member in various professional
bodies like ISTE, IEEE and IET and a Fellow of the Institution
of Electronics and Telecommunication Engineers (IETE).
Currently, she is working as Principal in VV College of
Engineering, Tisaiyanvilai.
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